As the size of transistors is miniaturized in high-tech CMOS (complementary MOS) devices, the thickness of gate insulation films is reduced. This induces a problem of increased gate leak current. A technology has been studied to reduce the gate leak current by using a material having a high dielectric constant (high-k material) for the gate insulation film to increase the physical thickness thereof. Oxides containing Hf, Zr, Al, Ta, or the like may be cited as typical examples of the high-k materials. Another technology has recently been studied to improve the mobility of carriers, improve the on-current, and reduce the GIDL (Gate Induced Drain barrier Lowering) by a method in which a high-k material is inserted into an interface between a gate electrode and SiO2 or SiON forming a gate insulation film so that the transistor threshold voltage is adjusted according to a deposition density of the inserted high-k material so as to reduce the impurity concentration in the channel region. In this method, a deposition density of the metal element forming the high-k material required to set the transistor threshold voltage to an appropriate value is desirably 1E14 (or 1×1014) atoms/cm2 or lower.
A film of the above-mentioned high-k material may be formed by diverse film formation methods such as sputtering method, CVD (Chemical Vapor Deposition) method, and atomic layer adsorption/deposition method. However, the sputtering method entails a fear that gate insulation film may be affected by plasma damage during the film formation. The CVD method eliminates the effect of plasma damage being a cause for the problem in the sputtering method, and is suitable for formation of a film of a mixture of the high-k material and silicon. However, there exists in the film formation process an incubation time in which the film thickness and the film formation time are not proportional. This leads to a problem of controllability, in-plane uniformity, and reproducibility of the film thickness. Additionally, it becomes difficult to ensure the in-plane uniformity or reproducibility of the film thickness depending on process conditions such as film formation temperature or raw material gas distribution. As for the atomic layer adsorption/deposition method, in contrast, its film formation mechanism utilizing saturated adsorption of a raw material is able to realize more favorable uniformity and reproducibility in comparison with the CVD method. Further, it is believed that the atomic layer adsorption/deposition method is most suitable for controlling the deposition density of the metal element since, in principle, the film formation in units of molecular layers is possible. However, even in the atomic layer adsorption/deposition method, there exists incubation time in which the film growth rate is low in an initial stage of the film formation. Therefore, the atomic layer adsorption/deposition method requires improvement in the controllability, reproducibility, and in-plane uniformity of the metal element deposition density.
For example, Non-Patent Document 1 (Journal of Applied Physics, Vol. 92, No. 12, 15 Dec. 2002, pp. 7168-7174) describes formation of a HfO2 film by an atomic layer adsorption/deposition method, in which the deposition density of Hf element per cycle is 1.26E14 atoms/cm2, and the film formation rate is 0.5 Å per cycle. It also describes that the deposition density of Hf element differs significantly depending on the surface state of a substrate to be treated. For example, comparing the cases in which a silicon substrate has a chemical oxide film and it has a thermal oxide film on the surface thereof, the Hf deposition amount per cycle on the one with the thermal oxide film is about a half of that on the one with the chemical oxide film. This phenomenon is particularly remarkable in the initial stage of the film formation (in a region where the number of cycles is 20 or less) to impair the linearity of the Hf deposition density to the number of cycles, which proves the existence of the incubation time.
Patent Document 1 (Japanese Laid-Open Patent Publication No. 2004-79753) discloses a technique in which atomic-layer adsorption/deposition of HfO2 is performed by using tetrakis(diethylamino)hafnium (Hf[N(C2H5)2]4) as a raw material and O3 as an oxidizing agent and alternately supplying them onto a silicon substrate. In this technique, a film formation rate of 0.8 Å per cycle is realized. An in-plane uniformity of film thickness of about 7% is realized. The in-plane uniformity is computed by (maximum measurement value-minimum measurement value)/(average measurement value×2)×100(%).
Further, Non-Patent Document 2 (Journal of Vacuum Science Technology, A23(3), May/June 2005, L1-L3) reports that atomic-layer adsorption/deposition of HfxSi(1−x)O2 is performed by using tetrakis(ethyl-methyl-amino)hafnium (Hf[N(C2H5) (CH3)]4) as an Hf raw material, tetrakis(ethyl-methyl-amino)silicon (Si[N(C2H5) (CH3)]4)as an Si raw material, and O3 as oxidation gas, and alternately performing a simultaneous supply of the Hf raw material gas and the Si raw material gas and a supply of O3, and a film formation rate of 0.8 Å per cycle has been realized.
Patent Document 2 (Japanese Laid-Open Patent Publication No. 2003-347297) discloses a technique in which atomic-layer adsorption/deposition of HfxSi(1−x)O2 is performed by using tetrakis(dimethyl-amino)hafnium as an Hf raw material, tetramethoxy-silane (Si(O CH3)4) as an Si raw material, and an oxidizing agent, and supplying the Hf raw material, the oxidizing agent, the Si raw material, and the oxidizing agent sequentially in this order onto the substrate in each cycle. In this case, a film formation rate of 2 Å per cycle is realized.
Patent Document 3 (Japanese Laid-Open Patent Publication No. 2002-151489) discloses an atomic layer adsorption/deposition method for forming a ZrSiO4 film, in which ZrCl4 and SiCl4 are used as raw material gases while H2O is used as an oxidation gas, and a ZrCl4 supply step, a purge step, an H2O supply step, a purge step, an SiCl4 supply step, a purge step, and an H2O supply step are sequentially performed as one cycle to alternately deposit a ZrO2 molecular layer and an SiO2 molecular layer.
However, the film formation techniques above have problems as described below.
In the first place, it is difficult according to the conventional atomic layer adsorption/deposition methods to reduce the deposition density of metal elements per cycle due to their film formation mechanisms. Specifically, according to these conventional methods, one molecular layer is formed per cycle and, therefore, the deposition density cannot be made lower than the deposition density of the molecular layer. For example, the HfO2 film formation rate per cycle in the above-mentioned conventional examples is from 0.5 Å to 0.8 Å. In that case, the Hf deposition density per cycle is from 1.26E14 atoms/cm2 to 1.8E14 atoms/cm2. Accordingly, it is impossible to obtain the above-mentioned favorable deposition density per cycle of 1E14atoms/cm2 or low.
In the second place, according to the conventional atomic layer adsorption/deposition methods, the incubation time exists depending on the surface state or the like of a substrate to be treated, and hence the deposition amount per cycle varies. This induces a problem of difficulty in ensuring the controllability, in-plane uniformity, and reproducibility of the deposition density.
In the third place, according to the conventional CVD methods, the film thickness varies due to the existence of the incubation time, which induces a problem of deterioration of the controllability, in-plane uniformity, and reproducibility of the film thickness.
In the fourth place, the metal element deposition density may be reduced in molecular layer units by a method of forming a film of a mixture of the metal element and Si element by means of the atomic layer adsorption/deposition method. However, for example, when an HfxSi(1−x)O2 is formed by the atomic layer adsorption/deposition method by supplying the Hf raw material and the Si raw material simultaneously, the Hf and Si atoms are deposited on the substrate to be treated by competitive adsorption between them. Therefore, it is difficult to ensure the reproducibility and uniformity of the composition.
In the fifth place, the incubation time exists also when a hafnium silicate film is formed by a method in which an Hf raw material, an oxidizing agent, an Si raw material, and an oxidizing agent are sequentially supplied onto a substrate to be treated to perform atomic-layer adsorption/deposition in one cycle. This makes it difficult to ensure the controllability of deposition density and the reproducibility and in-plane uniformity of film thickness. In addition, the HfO2 and SiO2 layers are formed in a stacked structure on the surface of the substrate to be treated, which causes a problem that the Hf deposition concentration cannot be reduced in the units of molecular layers. Further, according to this supply method, the thickness of the stacked SiO2 film must be increased to reduce the density of Hf element in the film. As a result, the EOT (effective oxide thickness) of the gate insulation film as a whole is increased.
It is an object of the present invention to reduce the incubation time, and to thereby suppress the variation in film thickness, improve the in-plane uniformity and reproducibility, and to enhance the controllability of metal element deposition density.